The present invention is directed to resonators for oscillator circuits and more specifically to resonators for use in low radiation oscillators.
Zero intermediate frequency (ZIF) or very low intermediate frequency (IF) architecture is taking the place of heterodyne conversion architecture in many wireless applications. ZIF architecture provides low part count, simple frequency plan, and immunity to system noise (such as digital switching noise or spurious noise).
However, ZIF architecture has a major disadvantage, both in receive and transmit chains because the local oscillator (LO) frequency is very close to that of the signal on the antenna (both received and transmitted). Leakage of the LO in the receive case generates a direct current (DC) level in the receiver output. This DC level is not static, and requires increased dynamic range of an analog to digital converter or sophisticated cancellation techniques in some standards (such as Global System for Mobile Communications (GSM)). In the transmit case, the LO leakage may override the signal transmitted to the antenna (such as in wireless code division multiple access (W-CDMA) systems where transmit levels may reach down to −40 decibels (dBm)). In any transmit case, there may be a problem of LO pulling by the transmitted signal, due to radiation coupling.
While certain techniques are used to reduce these phenomena, certain drawbacks exist. Frequency division or multiplication is the most common technique, however this results in a higher current drain and circuit area and a limited noise floor. Other techniques utilize polar feedback to cope with transmitter effects, and cause the same current/area (and therefore cost) penalty.
Thus there is a need for a local oscillator with lower radiation to prevent leakage and pulling.